228 FIELD COLUMBIAN MUusEUM—GEOLOGY, VOL. II. 
Again the derivation can hardly be ascribed to ferrous silicates of the 
adjoining rocks as in the case of the Lake Superior ores, since such 
silicates seem to be practically lacking in the associated rocks here. 
On the whole, therefore, the relation, either in time or manner of origin, 
between the associated eruptive rock and the iron oxide, and the 
origin of the iron oxide itself. seem as yet difficult to determine with 
certainty. 
